There are many ways in which motion may be estimated between two images. This motion may be described by a set of motion parameters that describe motion of luminance of pixels from a first image to a second image. These motion parameters may be defined at a time associated with either or both of the first and second images, or may be defined at a time between the first and second images. Thus, a vector for each pixel describes the motion of the pixel from one image to the next.
This motion estimate may be computed by using a gradient-based method, of which an example is a technique referred to as computing the “optical flow” between the images, or by using a correlation-based method. The “constant brightness constraint,” the assumption underlying the computation of optical flow, may be violated because of a change in an object's position relative to light sources, an object's specularity, an overall luminance change, a lack of similarity between the input images, an object entering or leaving a scene, or an object becoming revealed or occluded. If a violation of the constant brightness constraint occurs, the motion vectors from one coherent area of motion may spread into another coherent area of motion.
If erroneous motion information is used to perform various image processing operations, such as morphing, warping, interpolation, motion effects, and motion blurring, visible artifacts are seen in the resulting output images. For example, in some cases, the foreground may appear to stretch or distort, or the background may appear to stretch or distort, or both.